The present invention relates to a method to allocate data bits of digital data packet to a set of carriers in a multicarrier transmission system wherein a set of carriers, after being modulated with the data bits, is transmitted from a transmitter from a transmitter to a receiver via a transmission link, a subset of the set of carriers having frequencies within predetermined frequency ranges (Amateur Radio Band) with high probability for being affected by narrowbanded interference compared to carriers having frequencies outside the predetermined frequency ranges (Amateur Radio Band). It is also directed to a multicarrier transmitter adapted to transform a sequence of digital data packets into multicarrier data symbols and to apply the multicarrier data symbols via an output terminal to a transmission link to be transmitted thereover. It is also direct to a multicarrier receiver adapted to transform multicarrier data symbols received from a transmission link via an input terminal into a sequence of digital data packets. It is still further directed to an allocation message generator adapted to generate an allocation message to be communicated between a multicarrier transmitter and a multicarrier receiver in a multicarrier transmission system wherein digital data packets are transmitted between the multicarrier transmitter and the multicarrier receiver via a transmission link after being modulated on a set of carriers, a subset of carriers constituting the set of carriers having frequencies within frequency ranges (Amateur Radio Band) with high probability for being affected by narrowband interference (RFI) compared to carriers having frequencies outside these frequency ranges (Amateur Radio Band), said allocation message.
Such a method to allocate data bits and equipment adopted to perform such a method are already known in the art, e.g. from the article xe2x80x98A Multicarrier E1-HDSL Transceiver System with Coded Modulationxe2x80x99 from the authors Peter S. Chow, Naofal Al-Dhahir, John M. Cioffi and John A. C. Bingham. This article was published in the issue No. 3, May/June 1993 of the Journal of European Transactions on Telecommunications and Related Technologies (ETT), pages 257-266. Therein, a multicarrier transceiver system is described wherein digital data are modulated via Discrete Multi Tone (DMT) modulation on a set of carriers to be transmitted from a DMT transmitter to a DMT receiver via copper telephone lines. The block schemes of the DMT transmitter and DMT receiver are drawn in FIG. 4 and FIG. 5 on page 261 of the cited article respectively. In the DMT transmitter a bit allocation means, called a data bit encoder, allocates data bits of a digital data packet, called a block symbol, to the different carriers. The data bit encoder thereto uses formula (7) on page 260 of the article. A modulation means, i.e. the inverse fast Fourier transformer of FIG. 4, then modulates the data on the carriers where they are allocated to, to constitute the multicarrier symbols that are transmitted over the copper telephone line. FIG. 4A illustrates a possible constellation of data bits amongst carriers obtained by applying the known method. At the receiver""s side, a fast Fourier transformer demodulates these multicarrier symbols, and the decoder which forms part of the DMT receiver drawn in FIG. 5 of the above mentioned article, retrieves from each carrier the exact amount of data bits modulated thereon and thus performs the role of bit de-allocation means. This de-allocation means obviously has to know how many bits are modulated on each one of the carriers so that it can easily retrieve the exact amount of data bits from each carrier. In the known system, the bit de-allocation means gets this knowledge during initialisation of the transceiver system. Indeed, as is stated on page 263, in lines 22-30 of the left column, the DMT transmitter and DMT receiver negotiate with respect to bit and energy allocation during initialisation. As is understood from paragraph 2.2 of the article of Peter S. Chow et al., more particularly from lines 28-34 in the right column on page 259, certain carrier frequencies may be plagued by narrowbanded or single-frequency disturbances. In FIG. 4A such a disturbance is represented by RFI. Forward error correction techniques, well-known in the art, can reduce the effect of such disturbances. Nevertheless, unrecoverable errors may still appear at the receiver""s side. Thereto, Peter S. Chow et al. propose in their article the bitswapping solution: bit- and energy allocations are updated so that data bits are no longer transmitted via affected carriers. Such a re-allocation of data bits requires an additional communication between the DMT transmitter and DMT receiver, similar to the communication performed during initialisation, since both have to get aware of the new bit-allocations. Such a communication may be time-consuming and data bits may already be lost before the bits are swapped to less noisy carriers. Bitswapping thus may imply unrecoverable loss of information if it is seen as a solution for narrowbanded interference.
A problem similar to the just described one is known from the article xe2x80x98Overlapped Discrete Multitone Modulation for High Speed Copper Wire Communicationsxe2x80x99 from the authors Stuart D. Sandberg and Michael A. Tzonnes, an article which is published in the xe2x80x98IEEE Journal on Selected Areas in Communicationsxe2x80x99, Vol. 73, No. 9, December 1995 on pages 1571-1585 thereof. This article also describes a Discrete Multi Tone (DMT) modulation system which differs from the system described by Peter S. Chow et al. in that wavelet modulation and demodulation techniques are used instead of Fourier transforms. The wavelet transformation is, similar to the Fourier transformation, a linear transformation which transforms a time domain vector into a vector in another domain. This other domain is defined by its base functions which are complex exponentials for the Fourier transformation, and which can be more complex functions, implemented by means of general filter banks such as the cosine modulated filter bank, of another wavelet transformation. More details with respect to the wavelet transformation are found in the book xe2x80x98Numerical Recipes in Cxe2x80x99, written by William H. Press, Saul A Teukolsky, William T. Vetterling and Brian P. Flannery and published by the Cambridge University Press, in paragraph 13.10 on pages 591-606 entitled xe2x80x98Wavelet Transformsxe2x80x99. As mentioned in the left column on page 1583 of the article of Sandberg and Tzannes, their multicarrier system may be disturbed by narrowbanded interference due to the presence of radio frequency signals. In other words, the transmission line may pick up signals broadcasted by radio amateur transmitters as a result of which some carriers in the multicarrier data symbols transported by this transmission line may be damaged. In their article, Sandberg and Tzannes prove that their system, thanks to the wavelet modulation and demodulation techniques, has an improved immunity for such narrowbanded interference compared to multicarrier systems using Fourier transform modulation and demodulation methods, due to the intrinsic better spectral containment of the carrier waveforms. Nevertheless, also in the system of Sandberg and Tzannes, unrecoverable errors still have to be solved by re-allocation of data bits.
An object of the present invention is to provide a method for allocating data bits to a set of carriers and related equipment of the above known type, but wherein unrecoverable loss of information due to narrowbanded interference and time-consuming communications of the above described type are avoided.
According to the invention, this object is achieved by a method to allocate data bits of digital data packets to a set of carriers in a multicarrier transmission system wherein the set of carriers, after being modulated with the data bits, is transmitted from a transmitter to a receiver via a transmission link, a subset of the set of carriers having frequencies within predetermined frequency ranges (Amateur Radio Band) with high probability for being affected by narrowbanded interference compared to carriers having frequencies outside the predetermined frequency ranges (Amateur Radio Band) wherein at least part of the data bits of the digital data packets that are allocated to carriers of the subset of carriers, are allocated in a redundant way.
The object is also achieved by a multicarrier transmitter adapted to transform a sequence of digital data packets into multicarrier data symbols and to apply the multicarrier data symbols via an output terminal to a transmission link to be transmitted thereover, the multicarrier transmitter including between an input terminal and an output terminal the cascade connection of a bit allocation means, adapted to allocate data bits of the digital data packets to carriers of a set of carriers whereon the data packets have to be modulated, a subset of a set of carriers having frequencies within predetermined frequency ranges (Amateur Radio Band) with high probability for being affected by narrowbanded interference compared to carriers having frequencies outside the predetermined frequency ranges (Amateur Radio Band); and modulation means adapted to modulate the data bits on the carriers where they are allocated to, to thereby constitute the multicarrier data symbols, wherein the bit allocation means is adapted to allocate data bits in a redundant way to the carriers in the subset of carriers having frequencies within the predetermined frequency ranges (Amateur Radio Band) with high probability for being affected by narrowbanded interference.
The object is still further achieved by a multicarrier receiver adapted to transform multicarrier data symbols received from a transmission link via an input terminal into a sequence of digital data packets, the multicarrier receiver including between the input terminal and an output terminal thereof the cascade connection of: demodulation means, adapted to demodulate the multicarrier data symbols from a set of carriers where they are modulated on, a subset of the set of carriers having frequencies within predetermined frequency ranges (Amateur Radio Band) with high probability for being affected by narrowbanded interference compared to carriers having frequencies outside these predetermined frequency ranges (Amateur Radio Band); and bit de-allocation means, adapted to retrieve from each carrier of the set of carriers the exact number of data bits that was modulated thereon, wherein the multicarrier receiver further includes: narrowbanded interference measurement means, adapted to measure for each carrier in the subset of carriers the amount of narrowbanded interference by which the carrier is affected; diversity means, an input of which is coupled to an output of the narrowbanded interference measurement means and respective outputs of which are coupled to a control input of the demodulation means and a control input of the bit de-allocation means, and adapted to decide which data bits amongst redundantly allocated data bits are taken for demodulation and re-combination; and further wherein: the demodulation means is adapted to demodulate the data bits taken by the diversity means; and the bit de-allocation means is adapted to retrieve and recombine the data bits taken by the diversity means.
The object is also achieved by an allocation message generator, adapted to generate an allocation message to be communicated between a multicarrier transmitter and a multicarrier receiver in a multicarrier transmission system wherein digital data packets are transmitted between the multicarrier transmitter and the multicarrier receiver via a transmission link after being modulated on a set of carriers, a subset of carriers constituting the set of carriers having frequencies within frequency ranges (Amateur Radio Band) with high probability for being affected by narrowbanded interference compared to carriers having frequencies outside these frequency ranges (Amateur Radio Band), the allocation message generator including: a carrier identifier generator, adapted to generate a first parameter referring to one carrier of the set of carriers where the allocation message is related to; a bit amount generator, adapted to generate a second parameter representing an amount of data bits that is allocated in the multicarrier transmitter to the carrier where the allocation message is related to; a power amount generator, adapted to generate a third parameter representing a power level at which the carrier where the allocation message is related to, is transmitted; and embedding means, respective inputs of which are coupled to outputs of the carrier identifier generator, the bit amount generator, and the power amount generator respectively, the embedding means being adapted to embed the first parameter, the second parameter and the third parameter in respective fields of the allocation message, wherein the allocation message generator further includes: a redundancy parameter generator, adopted to generate a fourth parameter indicating whether the carrier where the allocation message is related to, carries redundant information or not; and further wherein the embedding means is provided with an additional input whereto an output of the redundancy parameter generator is coupled, the embedding means being adapted to also embed the fourth parameter in a respective field of the allocation message.
Indeed, narrowbanded or single frequency interference such as due to radio amateur signals may swap from one frequency to another, but, in accordance to certain specifications, always stays within predetermined frequency ranges. As a result, only a limited number of carriers, those having frequencies within these specified ranges, may be affected by the narrowbanded interference. This limited number of carriers constitutes a subset of carriers which, according to the present invention, is protected by modulating data thereon in a redundant way. The same data bits may for instance be modulated two or three times on carriers which form part of the subset. Alternatively, a linear or more complex combination of data bits modulated on certain carriers may be modulated on other carriers. One can even think of an implementation of the present invention wherein data bits of some multicarrier data symbols are combined in a linear or more complex way, and wherein the combined data symbols are sent on other carriers as part of other multicarrier data symbols. The multicarrier receiver then can retrieve data bits which are unrecoverably affected from other carriers if one of the carriers is damaged. No additional communication between the multicarrier transmitter and receiver is required. The multicarrier receiver just has to be aware of the redundancy scheme used by the transmitter. In other words, the receiver has to know which data bits are duplicated on which carriers in the subset or how data bits were combined and modulated on other carriers. This is told to the receiver by simple allocation messages which are generated for each carrier by the allocation message generator according to the present invention. These messages are all transmitted during the initialisation procedure. Obviously, the data recovery improves if the level of redundancy increases, but, as will be explained later on, an increased redundancy level inevitably reduces the throughput capacity of the transmission line. Thus, there is a trade-off between narrowbanded interference immunity by redundant transmission of data and throughput capacity of the line.
An additional feature of the present invention is that, in a first low-complexity implementation thereof, each data bit allocated to a carrier of the subset that may be affected, is in copy allocated to another carrier of the subset.
Thus, whenever a data bit is unrecoverably affected by narrowbanded interference, a copy of this data bit can still be obtained by the receiver from another carrier. This is not valid for alternative implementations of the present method wherein for instance only important data bits modulated on carriers in the subset and not any data bit are given this level of protection.
Yet another feature of the present allocation method is that, still in this first particular implementation, the data bits allocated to carriers in the lower half of this subset are in copy allocated to carriers in the upper half of this subset.
In this way, the carriers to which data bits and their copies are allocated are separated over a certain distance on a frequency scale. The probability that both the data bits and their copies will be unrecoverably affected by narrowbanded interference is reduced compared to implementations of the present invention wherein carriers with neighboring frequencies carry the data bits and their copies. It has however to be noted that this implementation, wherein the carriers are chosen, one in the lower half and one in the upper half of the frequency bands, may not always be recommended, since it assumes that the carriers in the upper- and lower half of this subset more or less have the same capacity for carrying data bits. This capacity of a carrier is the maximum number of data bits that, given the power level at which the carrier is transmitted and the signal to noise ratio of the carrier measured at the receiver""s side, is allowed to be allocated to this carrier. If the carriers in the upper half and lower half of the subset have great differences in capacities, part of the capacity of this half of the subset with the highest capacity for carrying data bits will not be used. A disadvantageous consequence thereof is a decreased throughput of the transmission line.
Furthermore, a feature of the present invention is that, still in this first implementation thereof, data bits allocated to a carrier with index k in the subset are in copy allocated to a carrier with index T+k, T being half the number of carriers constituting the subset.
In this way, the receiver can obtain all data bits from one single other carrier if a carrier is affected by narrowbanded interference and does not have to collect copies of the affected data bits from a plurality of other carriers. A person skilled in the art will recognise that the receiver complexity is reduced significantly in this case. In addition, the probability that both carriers carrying the data bits and their copies respectively, will be affected by the same narrowbanded interference, is minimised since there are always at least Txe2x88x92l carriers in the set of carriers between these two carriers. Similar to what is explained already above, the capacity of the transmission line may be inefficiently used if carriers associated with each other do not have capacities for carrying data bits that are more or less equal.
Moreover, an additional feature of the present invention is that, in a second implementation thereof, carriers in the subset may be associated with each other, group by group, all carriers in one group carrying the same data bits.
Thus, the more carriers constitute one group and are modulated with the same data bits, the higher the protection against narrowbanded interference, however, the lower the throughput capacity of the transmission line. A person skilled in the art will understand that carriers constituting one group have to be selected carefully: their frequencies should be spread over a wide frequency range to have good protection, and their capacities for carrying data bits should be comparable so as to minimise throughput loss.
A third implementation of the present method uses a combination of data bits that are allocated to carriers in the subset of carriers and are also allocated to other carriers.
In this way, by combining data bits and transmitting the combination of the data bits over other carriers than those where the original data bits are sent over, redundant transmission can be achieved with a higher throughput capacity than by duplicating or triplicating the data bits. This advantageous throughput capacity however will be paid by a higher complexity in the receiver. The receiver has to be able of re-generating the original data bits from the different versions thereof that were sent.
An additional feature of the third implementation is that redundant information, such as combinations of data bits modulated on certain carriers, may be transmitted as part of later transmitted multicarrier data symbols, so that the probability of being damaged by interference is even more reduced.
An additional advantageous feature of the third implementation is, that.
Indeed, when data bits are linearly combined, the inverse operation also is a linear combination of the received data bits as a result of which the receiver complexity can be kept low.
Yet another feature of the present method is that the degree of redundancy for transmission of data bits on carriers in the affected range may depend on quality of service requirements.
In this way, data bits of services with strong bit error protection requirements, such as financial data transmission, or sensitive OAM (Operation And Maintenance) information like synchronisation information, with strong bit error requirements, may be sent with a high degree of redundancy so that their transmission is well-protected. The transmission of data bits of services such as speech or video on demand on the other hand is subjected to less severe bit error requirements. Consequently, to optimally use the capacity of the transmission line, it may be decided not to sent such data bits in a redundant way.